1. Field of the Invention
The invention relates to acoustic flowmeters including ultrasonic flowmeters used to measure flowing exhaust gas velocity.
2. Background Art
Transmitting pulses of acoustic energy through a fluid is useful for measuring the state and properties of the fluid, specifically the velocity and temperature. Piezoceramic elements are commonly used in acoustic transducers to generate ultrasonic acoustic pulses or continuous wave fields.
The ultrasonic flowmeter measures the flowing exhaust gas velocity using the relation:
      (                  1                  t          1                    -              1                  t          2                      )    =            (                        2          ⁢          V                L            )        ⁢          cos      ⁡              (        Φ        )              ⁢                  ⁢    where  
t1=ultrasonic pulse transit time upstream;
t2=ultrasonic pulse transit time downstream;
V=velocity of the gas through the pipe;
φ=angle between the ultrasonic beam and the pipe; and
L=path length of the ultrasonic beam between the transmitter and receiver transducers.
The transit time (i.e. t1 and t2) is measured by sending a pulse of ultrasound across the pipe, and measuring the time differential (i.e. transit time) for the pulse to be detected at the receiving transducer. With a gas flow velocity of 0 ft/s, a typical transit time is about 220 μs with a 3.1 inch path length. At maximum flow, the gas velocity is about 150 ft/s, with a resulting upstream transit time measuring about 256 μs, and a downstream transit time measuring about 34 μs.
The measured transit times are affected by the temperature of the gas. The speed of sound in air changes with temperature using the relation:c=(20.03)√{square root over (Tgas)} where 
c=speed of sound in meters per second; and
Tgas=absolute gas temperature in Kelvin.
At room temperature, Tgas is 298 K and c equals 345 m/s, or 1135 ft/s. With a high gas temperature of 425° C., Tgas is 698 K and c equals 529 m/s, or 1736 ft/s.
When the ultrasonic flowmeter is calibrated with a gas temperature of 298 K, the gas velocity is calculated by the relation:
  V  =            (                        1                      t            1                          -                  1                      t            2                              )        ⁢                  (                  L                      2            ⁢                          cos              ⁡                              (                Φ                )                                                    )            .      
When the gas temperature is increased to 698 K, we find that this velocity relation is 5% to 10% different than the true gas velocity.
Additional background information may be found in U.S. Pat. Nos. 5,756,360; 4,336,719; 5,217,018; 5,159,838; 6,343,511; 5,241,287; 4,743,870; 5,438,999; 4,297,607; and 6,307,302.
For the foregoing reasons, there is a need for an improved acoustic flowmeter calibration method.